Synchronous bus systems have been developed to support higher bandwidth digital systems. In a synchronous bus system, data packets are sent between a master device and one or more slave memory devices. The data packets travel in parallel with a system clock and maintain a precise phase to that clock. Typically, synchronous memory systems utilize a phase adjusting circuit on the master and on each one of the slave devices in order to align the on-chip receive and transmit clocks to maximize the reception timing margins both on the master and slave devices. U.S. Pat. Nos. 5,432,823 and 5,513,327, which are assigned to the assignee of the present invention and which are expressly incorporated by reference herein, describe synchronous memory systems with phase adjusting circuitry on both master and slave devices.
The primary disadvantage of using phase alignment circuitry in every slave device of a master/slave system is that it increases power dissipation. For example, if the “standby” power of phase alignment circuitry, such as a Delay Locked Loop, is 125 mW, a system with 32 slave devices on standby mode will dissipate a total of 4 W. This power dissipation is prohibitively large for emerging portable applications.
In view of the foregoing, it would be highly desirable to provide a master/slave system in which the slave devices do not require phase alignment circuitry, thereby allowing the master/slave system to operate with substantially reduced power dissipation.